Burner for operating a heat generator

ABSTRACT

In a burner for operating a combustion chamber, which burner essentially comprises a swirl generator (100), a transition piece (200) arranged downstream of the swirl generator, and a mixing tube (20), transition piece (200) and mixing tube (20) forming the mixing section of the burner and being arranged upstream of a combustion space (30). The swirl generator (100) itself comprises at least two hollow, conical sectional bodies (140, 141, 142, 143) which are nested one inside the other in the direction of flow, the respective center axes of these sectional bodies running mutually offset in such a way that the adjacent walls of the sectional bodies form inlet ducts (120), tangential in their longitudinal extent, for a combustion-air flow (115). In the region where the combustion-air flow (115) flows into the swirl generator (100), fuel injectors (116, 116a) are arranged on both sides along the inflow edges, which fuel injectors act offset with respect to one another, in such a way that the inflow cross section of the duct (120) is integrally covered with fuel, with the result that a maximized premixing is achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a burner for operating a heatgenerator.

2. Discussion of Background

EP-0 780 629 A2 has disclosed a burner which consists of a swirlgenerator on the incident-flow side, the flow formed herein being passedover smoothly into a mixing section. This is done with the aid of a flowgeometry, which is formed at the start of the mixing section for thispurpose and consists of transition passages which cover sectors of theend face of the mixing section, in accordance with the number of actingsectional bodies of the swirl generator, and run helically in thedirection of flow. On the outflow side of these transition passages, themixing section has a number of prefilming bores, which ensure that theflow velocity along the tube wall is increased. This is then followed bya combustion chamber, the transition between the mixing section and thecombustion chamber being formed by a jump in cross section, in the planeof which a backflow zone or backflow bubble forms. The swirl intensityin the swirl generator is therefore selected in such a way that thebreakdown of the vortex does not take place inside the mixing sectionbut further downstream, as explained above, in the region of the jump incross section.

Although this burner, compared with those from the prior art, guaranteesa significant improvement with regard to intensification of the flamestability, lower pollutant emissions, lower pulsations, completeburn-out, large operating range, good cross-ignition between the variousburners, compact type of construction, improved mixing, etc., it hasbeen found that the increasing demands placed on burner technology maygive rise to problems with regard to adequate premixing between the fueland the combustion air, with the result that the pollutant emissionscannot always be minimized to the desired extent. In this respect, inorder to counteract this, it would be necessary for the distance betweenthe fuel injection location and the flame front to be very long, whichin the case of a burner for operating a heat generator is not possiblefor spatial reasons and operating considerations.

SUMMARY OF THE INVENTION

Accordingly, one object of the invention, as in a burner of theaforementioned type mentioned at the beginning, is to propose novelmeasures which are able to improve the mixing quality of the fuel/airmixture.

To achieve this object, the fuel is injected in the swirl generator onboth sides along the inlet ducts through which the combustion air flowsinto the interior.

The essential advantages of the invention may be seen in the fact that,owing to the injection of fuel provided on both sides of the inletducts, an improved depth of penetration of the fuel into the combustionflow is achieved, leading to improved premixing between fuel andcombustion air.

Furthermore, according to the invention it is provided for the injectionlevels of the two fuel-injector rows which are arranged at thetransition to the interior of the swirl generator to increase from thetip toward the outlet of the swirl generator. As a result, the sectioncovered before the fuel injectors situated further downstream enter theswirl generator, is increased, leading to better premixing of theinjected fuel.

The subject matter of the invention is also especially suitable for usein the case of other burners in which the swirl generator at the sametime forms the premixing section of the burner. In particular, in thisconnection, reference is made to publication EP-0 321 809 B1, which isan integral part of the present description.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a burner designed as a premix burner and having a mixingsection downstream of a swirl generator,

FIG. 2 shows a schematic cross section through a four-shell swirlgenerator,

FIG. 3 shows a four-shell swirl generator in three-dimensional view,

FIG. 4 shows a configuration of the transition geometry between swirlgenerator and mixing section, and

FIG. 5 shows a breakaway edge for the spatial stabilization of thebackflow zone.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, allfeatures not essential for the direct understanding of the inventionhave been omitted, and the direction of flow of the media is indicatedby arrows, FIG. 1 shows the overall construction of a burner. At thehead of the burner, a swirl generator 100 is effective, theconfiguration of which is shown and described in more detail below inFIGS. 2 and 3. This swirl generator 100 is a conical body to which anentering combustion-air flow 115 is repeatedly admitted tangentially inthe circumferential direction, various injections 116, 116a for agaseous and/or liquid fuel being disposed in the region where thiscombustion air 115 flows in: in this respect, reference is made to thestatements made under FIGS. 2 and 3. Further injection of fuel can beeffected through a fuel nozzle 103 which is arranged centrally and atthe head side. Here too, it is possible to operate using a liquid and/orgaseous fuel. The swirl flow forming here, with the aid of a transitiongeometry provided downstream of the swirl generator 100, is passedsmoothly into a transition piece 200, in such a way that no separationregions can form in this zone. The configuration of this transitiongeometry is described in more detail under FIG. 4. On the outflow sideof this transition piece 200, the transition geometry being formedthereby is extended by a mixing tube 20, both parts forming the actualmixing section 220 of the burner. The mixing section 220 may of coursebe made in one piece; i.e. the transition piece 200 and mixing tube 20are then fused to form a single cohesive body, the characteristics ofeach part being retained. If transition piece 200 and mixing tube 20 aremade from two parts, these parts are connected by a sleeve ring 10, thesame sleeve ring 10 serving as an anchoring surface for the swirlgenerator 100 on the head side. In addition, such a sleeve ring 10 hasthe advantage that various mixing tubes can be used without having tochange the basic configuration of the burner in any way. Located on theoutflow side of the mixing tube 20 is the actual combustion space 30 ofa combustion chamber, which is shown here merely by a flame tube. Themixing section 220 largely fulfills the task of providing a definedsection, in which perfect premixing of fuels of various types can beachieved, downstream of the swirl generator 100. Furthermore, thismixing section, that is primarily the mixing tube 20, enables the flowto be directed free of losses so that at first no backflow zone orbackflow bubble can form even in interaction with the transitiongeometry, whereby the mixing quality for all types of fuel can beinfluenced over the length of the mixing section 220. However, thismixing section 220 has another property, which consists in the factthat, in the mixing section 220 itself, the axial velocity profile has apronounced maximum on the axis, so that a flashback of the flame fromthe combustion chamber is not possible. However, it is correct to saythat this axial velocity decreases toward the wall in such aconfiguration. In order also to prevent flashback in this region, themixing tube 20 is provided in the flow and peripheral directions with anumber of regularly or irregularly distributed bores 21 having widelydiffering cross sections and directions, through which an air quantityflows into the interior of the mixing tube 20 and induces an increase inthe rate of flow along the wall for the purposes of a prefilmer. Thesebores 21 may also be designed in such a way that effusion coolingappears at least in addition at the inner wall of the mixing tube 20.Another possibility of increasing the velocity of the mixture inside themixing tube 20 is for the cross section of flow of the mixing tube 20 onthe outflow side of the transition passages 201, which form thetransition geometry already mentioned, to undergo a convergence, as aresult of which the entire velocity level inside the mixing tube 20 israised. In the figure, these bores 21 run at an acute angle relative tothe burner axis 60. Furthermore, the outlet of the transition passages201 corresponds to the narrowest cross section of flow of the mixingtube 20. Said transition passages 201 accordingly bridge the respectivedifference in cross section without at the same time adversely affectingthe flow formed. If the measure selected initiates an intolerablepressure loss when directing the tube flow 40 along the mixing tube 20,this may be remedied by a diffuser (not shown in the figure) beingprovided at the end of this mixing tube. A combustion chamber(combustion space 30) then adjoins the end of the mixing tube 20, therebeing a jump in cross section, formed by a burner front, between the twocross sections of flow. Not until here does a central flame front havinga backflow zone 50 form, which backflow zone 50 has the properties of abodiless flame retention baffle relative to the flame front. If afluidic marginal zone, in which vortex separations arise due to thevacuum prevailing there, forms inside this jump in cross section duringoperation, this leads to intensified ring stabilization of the backflowzone 50. At the end face, the combustion space 30, provided thislocation is not covered by other measures, for example by pilot burners,has a number of openings 31 through which an air quantity flows directlyinto the jump in cross section and there, inter alia, helps to intensifythe ring stabilization of the backflow zone 50. In addition, it must notbe left unmentioned that the generation of a stable backflow zone 50requires a sufficiently high swirl coefficient in a tube. If such a highswirl coefficient is undesirable at first, stable backflow zones may begenerated by the feed of small, intensely swirled air flows at the tubeend, for example through tangential openings. It is assumed here thatthe air quantity required for this is approximately 5-20% of the totalair quantity. As far as the configuration of the burner front 70 at theend of the mixing tube 20 for stabilizing the backflow zone or backflowbubble 50 is concerned, reference is made to the description under FIG.5.

FIG. 2 shows a swirl generator 100, which is composed of four sectionalbodies 140, 141, 142, 143, these sectional bodies having a bladeprofile, thus bringing about controlled flow for the combustion-air flow115 flowing into the interior 114 through the respective inlet ducts120. The cross section of flow of the inlet ducts 120 is achieved byoffsetting the respective center axes 141a, 142a, 143a, 144a of thesectional bodies, as emerges particularly clearly from FIG. 2. The fuel116, 116a is injected in the swirl generator on both sides along theinlet ducts 120. A more detailed description of the type of injectionemerges from the statements made under FIG. 3.

FIG. 3 shows a perspective view of a four-slot swirl generator 100. Thefuel 116, 116a for mixing into the combustion-air flow 115 is in thiscase guided in by means of fuel lines which are integrated in thesectional bodies 140-143, in contrast to the fuel supply in accordancewith EP0 780 629 A2. The introduction of fuel along the inlet ducts 120on both sides is in this case designed in such a way that the individualinjections lying opposite one another are arranged axially offset withrespect to one another. As a result, the intermediate space between twoinjections on one side is filled by the opposite, offset injection onthe other side. This is important since, as a result, the injected fuel,which is caught by the combustion-air flow 115, forms a spray in theform of bubbles. Fuel bubbles which form on opposite sides and offsetfrom one another make it possible to fill the entire cross section ofthe inlet ducts 120, and the depth of penetration of the fuel fed in isgreater, which has a positive effect on the formation of thefuel/combustion air mixture. A further measure for optimally configuringthe formation of the mixture relates to the configuration of theinjection level H of the fuel 116, 116a in the axial direction of theswirl generator 100. This increases from the tip of the swirl generator100 toward the swirl generator outlet. As a result, the relativepremixing section for the fuel injections which are situated furtherdownstream of the swirl generator tip is increased, leading to theremixing process becoming more intensive. The described change caused bythe geometric profile 144, 145 of the injection levels in the axialdirection can be seen from this figure. Naturally, the swirl generatormay otherwise be designed in accordance with EP0 780 629 A2, thisdocument forming an integral part of the present description. Swirlgenerators having a different number of inlet ducts 120 are alsopossible.

FIG. 4 shows the transition piece 200 in a three-dimensional view. Thetransition geometry is constructed for a swirl generator 100 having foursectional bodies in accordance with FIGS. 2 and 3. Accordingly, thetransition geometry has four transition passages 201 as a naturalextension of the sectional bodies acting upstream, as a result of whichthe cone quadrant of said sectional bodies is extended until itintersects the wall of the mixing tube. The same considerations alsoapply when the swirl generator is constructed from a principle otherthan that described under FIG. 3. The surface of the individualtransition passages 201 which runs downward in the direction of flow hasa form which runs spirally in the direction of flow and describes acrescent-shaped path, in accordance with the fact that in the presentcase the cross section of flow of the transition piece 200 widensconically in the direction of flow. The swirl angle of the transitionpassages 201 in the direction of flow is selected in such a way that asufficiently large section subsequently remains for the tube flow up tothe jump in cross section at the combustion-chamber inlet in order toeffect perfect premixing with the injected fuel. Furthermore, the axialvelocity at the mixing-tube wall downstream of the swirl generator isalso increased by the abovementioned measures. The transition geometryand the measures in the region of the mixing tube produce a distinctincrease in the axial-velocity profile toward the center of the mixingtube, so that the risk of premature ignition is decisively counteracted.

FIG. 5 shows the breakaway edge already discussed, which is formed atthe burner outlet. The cross section of flow of the tube 20 in thisregion is given a transition radius R, the size of which in principledepends on the flow inside the tube 20. This radius R is selected insuch a way that the flow comes into contact with the wall and thuscauses the swirl coefficient to increase considerably. Quantitatively,the size of the radius R can be defined in such a way that it is >10% ofthe inside diameter d of the tube 20. Compared with a flow without aradius, the backflow bubble 50 is now hugely enlarged. This radius Rruns up to the outlet plane of the tube 20, the angle β between thestart and end of the curvature being <90°. The breakaway edge A runsalong one leg of the angle β into the interior of the tube 20 and thusforms a breakaway step S relative to the front point of the breakawayedge A, the depth of which is >3 mm. Of course, the edge runningparallel here to the outlet plane of the tube 20 can be brought back tothe outlet-plane step again by means of a curved path. The angle β'which extends between the tangent of the breakaway edge A and theperpendicular to the outlet plane of the tube 20 is the same size asangle β. The advantages of this design of this breakaway edge can beseen from EP-0 780 629 A2 under the section "SUMMARY OF THE INVENTION".A further configuration of the breakaway edge for the same purpose canbe achieved with torus-like notches on the combustion-chamber side. Asfar as the breakaway edge is concerned, this publication, including thescope of protection there, is an integral part of the presentdescription.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A burner for operating a heat generator, theburner comprising:a swirl generator for a combustion-air flow, saidswirl generator having an upstream end and a downstream end, a directionof flow extending from said upstream end toward said downstream endalong a burner axis, and a plurality of swirl generating inlet ductseach having two sides; means for injecting at least one fuel into thecombustion-air flow; a mixing section arranged downstream of the swirlgenerator, said mixing section including transition passages for passingdownstream a flow formed in the swirl generator; a mixing tube arrangeddownstream of the transition passages, the flow from the transitionpassages passing into the mixing tube; wherein the injecting meanscomprises a fuel injector row on each side of each swirl-generatinginlet duct; wherein each of the two rows of fuel injectors forms aninjection level which increases from the tip to the outlet of the swirlgenerator.
 2. The burner in accordance with claim 1, wherein the fuelinjectors for each swirl-generating inlet duct are arranged offset inthe direction of flow with respect to one another.
 3. The burner inaccordance with claim 1, wherein the swirl generator comprises at leasttwo hollow, conical sectional bodies which are nested one inside theother in the direction of flow, each sectional body having walls and acenter axis, wherein the respective center axes of the sectional bodiesare mutually offset in such a way that adjacent walls of the sectionalbodies form the swirl-generating inlet ducts for the combustion-airflow, and wherein the sectional bodies together form a premixing sectionin the interior space formed between the sectional bodies.
 4. The burnerin accordance with claim 3, further comprising a fuel nozzle arranged atthe upstream end of the swirl generator.
 5. The burner in accordancewith claim 3, wherein each of the sectional bodies have a blade-shapedprofile in cross section.
 6. The burner in accordance with claim 3,wherein the sectional bodies are nested spirally one inside the other.7. The burner in accordance with claim 1, wherein the swirl generatinginlet ducts each form a partial flow when fluid flows therethrough, andwherein the number of transition passages in the mixing sectioncorresponds to the number of partial flows formed by the swirlgenerator.
 8. The burner in accordance with claim 1, wherein the mixingtube comprises openings extending at least partially in the direction offlow for injecting an air flow into the interior of the mixing tube(20).
 9. The burner in accordance with claim 8, wherein the openingsextend at an acute angle relative to the burner axis.
 10. The burner inaccordance with claim 1, further comprising a combustion space having across sectional dimension arranged downstream of the mixing section, themixing section having a cross sectional dimension different from thecombustion space cross sectional dimension, wherein the difference incross section between the mixing section and the combustion spacepermits a backflow zone to form in the combustion space.
 11. The burnerin accordance with claim 1, wherein the mixing tube has a downstream endand a breakaway edge on the downstream end.